EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



475 



5. Comparison reveals the general kinship oi Br ontops, 

 Allops, and Menodus in the evolution of the premolars. 



The most important biologic fact disclosed is that 

 the tetartocones show a somewhat different rate of 



v-yv^'^/-..V.^ J 



FiGUKB 407. — Progressive evolution of the lower pre- 

 molars in Brontolherium and its predecessors 



A, EotUanops princeps, Am. Mus. 290 (type), Wind River; B, Palaeosyops 

 paludosus, Am. Mus. 11680, lower Bridger (although this genus is not 

 In the direct line, its premolars well illustrate this stage of evolution) ; 

 C, Metarhinus earlei, Am. Mus. 13179, Washakie B; D, Broniotherium 

 medium, Am. Mus. 1051, Chadron C. All one-half natural size. 



evolution, or retardation and acceleration, in each 

 of these subphyla. A second fact of importance is 

 that Brontops in its later stages is convergent in many 

 characters with BrontotJierium. 



Ascending mutations between species. — That species 

 referred to Brontops and Diploclonus are allied 

 generically is shown by numerous skulls that com- 

 bine features of two species, either in the same 



Figure 408. — Progressive evolution of the 

 lower premolars in Brontops and its pred- 

 ecessors 

 A, EotUanops princeps, Am. Mus. 296 (type), Wind Riv- 

 er; B, Manteoceras manteoceras. Am. Mus. 1556, upper 

 Bridger; C, Protitanotherium emarginatum Princeton 

 Mus. 11242, Uinta C; D, Brontops brachycephalus. Am. 

 Mus. 1495, Chadron A. All one-half natural size. 



or in different phyla, such as B. hracTiycephalus 



and B. dispar (Nat. Mus. 4258, 1214) or D. bicor- 



nutuS) D. amplus, and type of D. tyleri, Amherst 

 Museum. 



ITie Brontops-Diploclonus phylum as represented in the Hatcher collection oj 60 sJculls and jaws from the Chadron 

 formation, in the United States National Museum 



